1. Field of the Invention
The present invention relates to a method for measuring the wettability of rock samples by measuring the relaxation time by low-field nuclear magnetic resonance.
2. Description of the Prior Art
There are well-known methods for determining the wettability of rocks to contained water and oil, comprising carrying out rock drainage cycles, that is displacement of fluids intended to decrease the water saturation, followed by imbibition. Imbibition relates to a displacement of fluids allowing an increase in the water saturation (Sw) of the rock. The capillary pressure Pc at one point is defined as the difference at equilibrium between the pressure P(oil) of the oil and the pressure P(water) of the water. This parameter is useful only if the two fluids are in the continuous phase in the porous medium. For a water wet medium, only the positive values are useful. On the other hand, when the medium has a mixed wettability, the fluids can remain in the continuous phase for the positive as well as for the negative capillary pressures (Pc).
For an application of this type, a complete capillary pressure measuring cycle generally comprises (FIG. 1):
a) positive primary drainage of an initially 100% water-saturated sample (curve 1);
b) positive imbibition (curve 2);
c) negative imbibition (curve 3);
d) negative drainage (curve 4); and
e) positive secondary drainage (curve 5); and wherein
f) Swir is the irreducible water saturation discussed below in the Practical implementation example, Sor is the residual oil saturation also discussed below in the Practical implementation example, and Ad and Ai are surface areas discussed below and are used in the definition of the wettability indices WI.
The curves represent evolution of Pc as a function of Sw with Swir being a specific value of SW.
There are various types of devices allowing the curves of FIG. 1 to be drawn.
In a device referred to as a “porous plate” device, notably described in U.S. Pat. No. 4,506,542, the porous rock sample containing two fluids in the continuous phase is placed in an elongate cell ended at its two opposite ends by capillary barriers permeable to a first fluid. This first fluid is injected under pressure through the first membrane and the pressure difference between the injection pressure and the pressure of the fluid discharged at the other end is measured. The pressures of the two fluids and the capillary pressure Pc are constant all along the sample, and the saturation is assumed to be uniform.
It is also known to carry out progressive-speed centrifugation by means of centrifugation devices such as those described for example in French Patent 2,772,477 corresponding to U.S. Pat. No. 6,185,985 or French Patent 2,763,690, or European Patent 603,040 corresponding to U.S. Pat. No. 5,463,894 or French Patent 2,798,734 filed by the Assignee.
The sample initially saturated with a first liquid (brine for example) is placed (FIG. 8) in an elongate vessel or cup A containing a second fluid of different density (oil for example). Cup A is fastened to the end of a rotating arm B and a centrifugal force is applied thereto so as to study the displacements of the fluids in the sample during at least two distinct steps. During a first drainage step, the assembly is then subjected to a centrifugal force applied along the length of the vessel so as to exert an expulsion force which tends to cause part of the first fluid to flow out. The pressure field created by centrifugation is expressed as a function of the density ρ, of the radius R and of the angular velocity w, by the relation: ½w2**·π(Rmax2−R2), for each fluid. The pressure of the two fluids at the sample outlet is the same and is equal to zero at the outlet. At the same time, the second fluid flows into the sample. The two fluids move in the sample until a position of equilibrium is reached where the force due to the capillary pressure in the pores compensates for the centrifugal force exerted. A measuring sonde is placed in the cup on the side of the sample. The sonde can be of capacitive type for example and comprise a metal rod insulated by a ceramic sheath. The capacity between the rod and the conducting fluid (brine), which is proportional to the immersed height, is measured. With this measuring system, the measuring accuracy is 1.5% of the pore volume. The sonde detects the position of the interface between the two liquids in the cup and transmits the measuring signals to a measuring signal acquisition and control automaton E including hydraulic liquid circulation means and an acquisition device.
During the re-imbibition stage, the velocity is decreased so as to study the return of the initial fluid into the sample. The local saturations measured with this type of device are calculated by an inversion program from the total amount of water expelled from the sample.
According to another method, referred to as a “dynamic” method, a sample is placed in an elongate cell having water-permeable membranes at its two ends. At a first end, oil under pressure is directly injected into the enclosure. Water is also injected, but this injection is carried out through the membrane and at a lower pressure. At the opposite end, the oil is directly discharged whereas the water flows out through the terminal membrane. Adjustment of the oil and water injection rates allows the capillary pressure to be the same at the inlet and at the outlet of the enclosure, which leads to a uniform saturation that can be deduced from the fluids balance. The capillary pressure is obtained for example by measuring the difference between the pressure of the oil and of the water at the enclosure outlet. Such a method is notably described by Brown H. W. in “Capillary Pressure Investigations”, Petroleum Transaction AIME, vol. 192, 1951. Examples of implementation are for example described in patents European Patent 729,022 corresponding to U.S. Pat. No. 5,698,772 or EP Patent 974,839 corresponding to U.S. Pat. No. 6,229,312 field by the Assignee.
A method referred to as semi-dynamic method is also known, wherein a rock sample imbibed with a first fluid is confined in a closed cell, another fluid under pressure is injected at a first end of the enclosure and the opposite end is swept by a low-pressure fluid circulated by a pump which carries the drained fluid outside. The device measures the pressure and the saturation of the sample, the amount of fluid discharged and the electric resistivity of the sample. This method is implemented for example in French Patent 2,708,742 corresponding to U.S. Pat. No. 5,679,885 filed by the Assignee.
Once drainage and imbibition curves are established, it is well-known to calculate the wettability indices WI from the surface areas Ad and Ai marked by the positive and negative capillary pressure curves, as shown in FIG. 1, by the relation
      WI          (      USBM      )        =      Log    ⁢                  ⁢                            A          d                          A          i                    .      